I will assume you're using the newest version of sabayon (or one of the more recent) and compiz fusion.
The defaults for zoom are i believe Super+ScrollWheelUp or Super+ScrollWheelDown (Super=windows key). If that doesn't work, right click the blue compiz fusion icon on the taskbar and go to settings, search for zoom, go under actions and unhide "general". You can doubleclick the appropriate keys there to change the key combination to whatever you want.
Hope this is what you were looking for. If not, repost.